This application claims the benefit of Japanese Application No. 2002-297076 filed Oct. 10, 2002.
The present invention relates to an MR (magnetic resonance) image producing method and an MRI (magnetic resonance imaging) apparatus, and more particularly to an MR image producing method and MRI apparatus that improve on rendering capability for a blood vessel.
A conventional MRI apparatus comprises MR signal acquiring means for acquiring MR signals, window-processing means for window-processing the MR signals using a window function that has a value of xe2x80x9conexe2x80x9d from the center of a k-space to a position near the periphery of the k-space and has a decreasing value as it goes closer to the periphery, and Fourier-transformation processing means for conducting Fourier-transformation processing on the window-processed MR signals to produce an MR image.
The window processing is conducted to concentrically suppress a high frequency portion of MR signals to thereby prevent truncation artifacts or anisotropic noise texture due to signal acquisition confined to a limited rectangular region in a k-space by the MRI apparatus.
Related conventional techniques are disclosed in Japanese Patent Application Laid Open Nos. H4-53531 and H6-121781.
In the conventional MRI apparatus, the same window processing is conducted whether the image to be produced is a blood flow image or not.
In other words, the conventional window processing is not optimal when the image to be produced is a blood flow image, and it cannot improve rendering capability for a blood vessel.
It is therefore an object of the present invention to provide an MR image producing method and MRI apparatus that improve on rendering capability for a blood vessel by optimizing window processing when an image to be produced is a blood flow image.
In a first aspect, the present invention provides an MR image producing method characterized in comprising: window-processing MR signals using a window function that has a value less than one at a center and in its proximate region in a k-space and on a periphery and in its proximate region in the k-space, and, between the regions in which the window function has a value less than one, has a value larger than that in the regions in which the window function has a value less than one; and applying Fourier-transformation processing to the window-processed MR signals to obtain an MR image.
In this configuration, the proximate region of the center of the k-space is a range of about fivexe2x80x94twenty data points from the center of the k-space. The proximate region of the periphery of the k-space is a range of about fivexe2x80x94twenty data points from the periphery of the k-space.
According to the MR image producing method of the first aspect, since a window function that has a value less than one at a center and in its proximate region in the k-space is employed, MR signals near the center of the k-space are suppressed. MR signals of a tissue portion are narrowly distributed near the center of the k-space, while MR signals of a blood flow portion are broadly distributed in a high frequency region, as well as near the center. Thus, the MR signals of the tissue portion are strongly suppressed, while the MR signals of the blood flow portion are relatively weakly suppressed. Therefore, rendering capability for a blood vessel is relatively improved.
Moreover, since the window function has a value less than one on a periphery and in its proximate region in the k-space, a high frequency portion of MR signals can be concentrically suppressed as in the conventional technique.
In a second aspect, the present invention provides an MR image producing method characterized in comprising: window-processing MR signals using a window function that has a value less than one at a center of a k-space, first increases to a value C equal to or more than one as it goes farther from the center, remains at C for a certain duration, then passes to one, and decreases to a value less than one as it goes from near a periphery to the periphery of the k-space; and applying Fourier-transformation processing to the window-processed MR signals to obtain an MR image.
In this configuration, the region in which the window function has a value increasing to a value C from the center of the k-space is a range of about threexe2x80x94fifteen data points from the center of the k-space. The region in which the window function remains at C for a certain duration is a range of about twentyxe2x80x94fifty data points. The region in which the window function passes from C to one is a range of about threexe2x80x94ten data points. The region in which the window function has a value less than one is a range of about fivexe2x80x94twenty data points from the periphery of the k-space.
According to the MR image producing method of the second aspect, since the window function has a value less than one at a center and in its proximate region in the k-space, MR signals near the center of the k-space are suppressed. MR signals of a tissue portion are narrowly distributed near the center of the k-space, while MR signals of a blood flow portion are broadly distributed in a high frequency region, as well as near the center. Thus, the MR signals of the tissue portion are strongly suppressed, while the MR signals of the blood flow portion are relatively weakly suppressed. Next, in the region in which xe2x80x9cthe window function remains at C for a certain durationxe2x80x9d, a zero-th order peak portion (a crest having a maximum value at the center) in the MR signals of the blood flow portion is preserved or amplified. Next, in the region in which xe2x80x9cthe window function passes to onexe2x80x9d, a first- or higher order peak portion (a crest having a maximum value at a position except the center) in the MR signals of the blood flow portion is preserved. Thus, rendering capability for a blood vessel is relatively improved.
Moreover, since the window function has a value less than one on a periphery and in its proximate region in the k-space, a high frequency portion of MR signals can be concentrically suppressed as in the conventional technique.
In a third aspect, the present invention provides the MR image producing method having the aforementioned configuration, characterized in that: the window function is a function using a Gaussian function in the region in which the window function increases to C.
According to the MR image producing method of the third aspect, a Gaussian function exp{xe2x88x92|k|2/a2} can be used to smoothly increase the value from a value less than one to a value C.
In a fourth aspect, the present invention provides the MR image producing method having the aforementioned configuration, characterized in that: the window function is a function using a Fermi-Dirac function in the region in which the window function decreases to a value less than one.
According to the MR image producing method of the fourth aspect, a Fermi-Dirac function 1/(1+exp{(|k|xe2x88x92R)/b}) can be used to smoothly reduce the value from one to a value less than one.
In a fifth aspect, the present invention provides an MR image producing method characterized in comprising: window-processing MR signals using a window function that has a value less than one at a center of a k-space, first increases to one as it goes farther from the center, remains at one for a certain duration, and decreases to a value less than one as it goes from near a periphery to the periphery of the k-space; and applying Fourier-transformation processing to the window-processed MR signals to obtain an MR image.
According to the MR image producing method of the fifth aspect, since the window function has a value less than one at a center and in its proximate region in the k-space, MR signals near the center of the k-space are suppressed. MR signals of a tissue portion are narrowly distributed near the center of the k-space, while MR signals of a blood flow portion are broadly distributed in a high frequency region, as well as near the center. Thus, the MR signals of the tissue portion are strongly suppressed, while the MR signals of the blood flow portion are relatively weakly suppressed. Next, in the region in which xe2x80x9cthe window function remains at one for a certain duration,xe2x80x9d MR signals of the blood flow portion are preserved. Thus, rendering capability for a blood vessel is relatively improved.
Moreover, since the window function has a value less than one on a periphery and in its proximate region in the k-space, a high frequency portion of MR signals can be concentrically suppressed as in the conventional technique.
In a sixth aspect, the present invention provides the MR image producing method having the aforementioned configuration, characterized in that: the window function is a function using a Gaussian function in the region in which the window function increases to one.
According to the MR image producing method of the sixth aspect, a Gaussian function exp{xe2x88x92|k|2/a2} can be used to smoothly increase the value from a value less than one to one.
In a seventh aspect, the present invention provides the MR image producing method having the aforementioned configuration, characterized in that: the window function is a function using a Fermi-Dirac function in the region in which the window function decreases to a value less than one.
According to the MR image producing method of the seventh aspect, a Fermi-Dirac function 1/(1+exp{(|k|xe2x88x92R)/b}) can be used to smoothly reduce the value from one to a value less than one.
In an eighth aspect, the present invention provides an MR image producing method characterized in comprising: producing MR images by the MR image producing method having the aforementioned configuration for a plurality of sequential slices; generating three-dimensional data from the MR images; and conducting MIP processing on the three-dimensional data to produce a projection image.
In the MR image producing method of the eighth aspect, rendering capability for a blood vessel can be improved for an angiographic image.
In a ninth aspect, the present invention provides an MRI apparatus characterized in comprising: MR signal acquiring means for acquiring MR signals; window-processing means for window-processing the MR signals using a window function that has a value less than one at a center and in its proximate region in a k-space and on a periphery and in its proximate region in the k-space, and, between the regions in which the window function has a value less than one, has a value larger than that in the regions in which the window function has a value less than one; and Fourier-transformation processing means for applying Fourier-transformation processing to the window-processed MR signals to obtain an MR image.
According to the MRI apparatus of the ninth aspect, the MR image producing method of the first aspect can be suitably implemented.
In a tenth aspect, the present invention provides an MRI apparatus characterized in comprising: MR signal acquiring means for acquiring MR signals; window-processing means for window-processing the MR signals using a window function that has a value less than one at a center of a k-space, first increases to a value C equal to or more than one as it goes farther from the center, remains at C for a certain duration, then passes to one, and decreases to a value less than one as it goes from near a periphery to the periphery of the k-space; and Fourier-transformation processing means for applying Fourier-transformation processing to the window-processed MR signals to obtain an MR image.
According to the MRI apparatus of the tenth aspect, the MR image producing method of the second aspect can be suitably implemented.
In an eleventh aspect, the present invention provides the MRI apparatus having the aforementioned configuration, characterized in that: the window function is a function using a Gaussian function in the region in which the window function increases to C.
According to the MRI apparatus of the eleventh aspect, the MR image producing method of the third aspect can be suitably implemented.
In a twelfth aspect, the present invention provides the MRI apparatus having the aforementioned configuration, characterized in that: the window function is a function using a Fermi-Dirac function in the region in which the window function decreases to a value less than one.
According to the MRI apparatus of the twelfth aspect, the MR image producing method of the fourth aspect can be suitably implemented.
In a thirteenth aspect, the present invention provides an MRI apparatus characterized in comprising: MR signal acquiring means for acquiring MR signals; window-processing means for window-processing the MR signals using a window function that has a value less than one at a center of a k-space, first increases to one as it goes farther from the center, remains at one for a certain duration, and decreases to a value less than one as it goes from near a periphery to the periphery of the k-space; and Fourier-transformation processing means for applying Fourier-transformation processing to the window-processed MR signals to obtain an MR image.
According to the MRI apparatus of the thirteenth aspect, the MR image producing method of the fifth aspect can be suitably implemented.
In a fourteenth aspect, the present invention provides the MRI apparatus having the aforementioned configuration, characterized in that: the window function is a function using a Gaussian function in the region in which the window function increases to one.
According to the MRI apparatus of the fourteenth aspect, the MR image producing method of the sixth aspect can be suitably implemented.
In a fifteenth aspect, the present invention provides the MRI apparatus having the aforementioned configuration, characterized in that: the window function is a function using a Fermi-Dirac function in the region in which the window function decreases to a value less than one.
According to the MRI apparatus of the fifteenth aspect, the MR image producing method of the seventh aspect can be suitably implemented.
In a sixteenth aspect, the present invention provides the MRI apparatus characterized in comprising: three-dimensional data generating means for generating three-dimensional data from MR images produced for a plurality of sequential slices; and MIP-processing means for conducting MIP processing on the three-dimensional data to produce a projection image.
According to the MRI apparatus of the sixteenth aspect, the MR image producing method of the eighth aspect can be suitably implemented.
According to the MR image producing method and MRI apparatus of the present invention, rendering capability for a blood vessel is improved. Moreover, noise in a high frequency region is suppressed to improve CNR (carrier to noise ratio).
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.